Ferritic stainless steel is used in broad fields such as kitchen equipment, home electrical appliances, electronic equipment, etc. However, it is inferior in formability compared with austenitic stainless steel, so the applications are sometimes limited. In recent years, improvements in refining techniques have enabled extreme reduction of carbon and nitrogen, reduction of Si, and also reduction of P, S, and other impurity elements. Ferritic stainless steel improved in formability by addition of Ti or other stabilizing elements (below, “high purity ferritic stainless steel”) is being used for an increasingly broader range of forming applications. This is because ferritic stainless steel is better in economy compared with austenitic stainless steel containing large amounts of Ni—which has been skyrocketing in price in recent years.
High purity ferritic stainless steel, as will be understood from SUS430LX standardized by the JIS, often has a lower amount of Cr compared with the typical austenitic stainless steel SUS304 (18Cr-8Ni) and has problems in corrosion resistance. For stainless steel sinks or other kitchen equipment or home electrical appliances where aesthetic appearance is demanded, deterioration of the surface properties due to pitting, corrosioning, or another corrosion is often a problem.
To improve the above corrosion resistance, there is the method of alloying Cr, Mo, etc. and the method of using bright annealing to modify the coating formed on the steel surface. The former invites a rise in cost due to the alloying and becomes a factor inhibiting formability, so is not preferable. The latter is a method effective from the viewpoints of suppressing the rise in the cost of materials and the drop in formability. Various inventions have been disclosed regarding the modification of coatings utilizing bright annealing.
From the latter viewpoint, PLT 1 discloses bright annealed finished ferritic stainless steel sheet excellent in corrosion resistance and formability which has a ratio of concentration of Cr/Fe in the coating of more than 0.5 and includes TiO2 in the coating and a method of production of the same. However, in steel utilizing bright annealing for modifying the coating, when forming and subsequent polishing/grinding causes new surfaces to be exposed, problems remain in securing corrosion resistance at the new surfaces. PLT 1 does not describe measures against these problems.
As means for solving the above problems, the method of utilizing trace amounts of elements to improve the corrosion resistance may be considered. PLT 2 and PLT 3 disclose ferritic stainless steel in which P is deliberately added to improve the weather resistance, corrosion resistance, and crevice corrosion resistance. PLT 2 discloses high Cr and P ferritic stainless steel containing Cr: over 20% to 40% and P: over 0.06% to 0.2%. PLT 3 discloses P ferritic stainless steel containing Cr: 11% to less than 20% and P: over 0.04% to 0.2%. However, P becomes a factor inhibiting manufacturability, formability, and weldability.
PLT 4 discloses ferritic stainless steel excellent in high temperature strength which includes trace elements of Sn and Sb and a method of production of the same. The majority of the steels shown in the examples of PLT 4 is low Cr steel of Cr: 10 to 12%. With high Cr steel of over Cr: 12%, to secure high temperature strength, V, Mo, etc. are added together. As effects of Sn and Sb, improvement of the high temperature strength is mentioned, but there is no description relating to the corrosion resistance.
PLT 5 describes a method of production of ferritic stainless steel sheet for automotive exhaust system-use excellent in deep drawability which adds one or more of Cu, Ni, W, and Sn. The steels shown in the examples of PLT 5 require that expensive Mo be added to 0.5% or more. As an effect of Sn, this is described as an element which improves the corrosion resistance in the same way as Cu, Ni, and W.
PLT 6 and PLT 7 disclose ferritic stainless steels excellent in surface properties and corrosion resistance which include Mg and Ca as trace elements and methods of production of the same. Sn is an optionally added element. It is described as an element preferable for corrosion resistance.
The steels shown in the examples of PLT 6 and PLT 7 have Sn and expensive Co added to them together. These steels are 11.6% Cr steel or 16% Cr steel containing large amounts of C or another impurity element. The pitting potential is described as being respectively 0.086 and 0.12V. This pitting potential does not in the end reach the corrosion resistance corresponding to SUS304 targeted by the present invention.
PLT 8 discloses ferritic stainless steel excellent in crevice corrosion resistance having Sn and Sb as trace elements for the purpose of improvement of the pitting life of auto parts etc. The steels shown in the examples of PLT 8 almost all have Sn and Ni added together for improving the pitting resistance at the crevice parts. 16% Cr steel to which Sn is added alone is high in amount of Si and does not correspond to the high purity ferritic stainless steel covered by the present invention.